Backtwisting and pre-twisting of conductors: types of stranding equipment and their features

Backtwist is a technological process in which the spool with the workpiece rotates in the opposite direction relative to the twisting element of the machine during one working cycle. This is necessary to reduce internal stresses in the wire that arise during the stranding of conductive cores (CC). Without backtwist, the wire may deform, increasing active resistance, causing losses, and potentially leading to wire breaks or displacement from the lay. Two types of mechanisms are used to perform backtwist: crank-eccentric and planetary mechanisms.

Crank-eccentric mechanism is a device designed for performing backtwist of wire during the stranding of conductive cores (CC). It consists of several key components: a backtwist ring, cranks, and supports (or cradles).

The backtwist ring is located behind the twisting section of the machine. Its axis is positioned lower than the axis of the main twisting mechanism, creating an eccentric offset. The cranks are pivotally connected to the backtwist ring and the axles of the supports (cradles). The backtwist ring itself does not have a physical axis of rotation – it is mounted on the cranks, which provide its movement.

When the twisting section of the machine begins to rotate, the cranks keep the supports (cradles) in a fixed position. This is achieved by the cranks compensating for the rotation of the twisting element, preventing the supports from shifting. As a result, the supports with the spools move in space parallel to their initial positions, and the wire, rotating around the axis of the stranded workpiece, does not twist around itself.

The process of moving the spool with the workpiece is as follows: during one full rotation of the twisting section, the spool rotates around the axis of the support (cradle) by 360° in the direction opposite to the rotation of the twisting mechanism. This movement ensures the compensation of internal stresses that arise during the stranding of the wire and prevents its deformation.

The planetary backtwist mechanism consists of a stationary central gear, a pinion gear, and a cradle gear. The central gear is fixed, and its axis coincides with the axis of the cage. The pinion gear, mounted on the twisting mechanism, rotates clockwise when the cage rotates. This movement is transferred to the cradle gear, causing it to rotate counterclockwise. As a result, the cradle with the spool completes a 360° rotation in the opposite direction, compensating for the internal stresses in the wire.

If the pinion gear is removed, the backtwist process stops, and the mechanism switches to the un-twisting mode. This allows for flexible control of the stranding process depending on the technological requirements.

Backtwist is performed only on cage-type machines.

Pre-twisting is performed only on cage-type machines. It is a technological operation that combines the compacting and twisting of the conductive core (CC) around its axis. The pitch of the backtwist corresponds to the pitch of the subsequent main stranding, which eliminates internal stresses and ensures the stability of the cable structure. Backtwist is carried out on twisting machines equipped with a backtwist device, which is integrated with the compacting rollers. The rollers rotate not only around their axis but also around the axis of the stranded core. This helps minimize internal stresses in the core, making it stable during the main stranding process.

Cage-type planetary stranding machine (named due to the planetary backtwist device).

Cage-type stranding machines are equipped with a planetary backtwist mechanism. The twisting section consists of a cage made of metal rings (discs) mounted on a hollow shaft. The supports with the spools are placed between the discs. Tension control is carried out in two ways: mechanical and magnetic.

Mechanical braking: is regulated using a brake band and a spring. When the spring is compressed, the contact area between the band and the brake disc increases, which enhances friction and tension. This method is used for larger cross-sections, where the risk of wire breakage under high tension is minimal. However, adjusting the tension requires stopping the machine.

Magnetic clutch: tension is regulated automatically and decreases as the spool empties. The control is carried out through a touch panel, allowing for tension adjustment without stopping the line. This method is used for smaller cross-sections, where the risk of wire breakage and stretching is high.

If there is one or more workpieces in the center of the stranded product, the spools with these workpieces are placed on feed mechanisms in front of the machine.

Cage-type machines are considered the most versatile among twisting equipment. They allow for helical stranding both with and without backtwist, as well as applying wire protection to cables. However, they have some drawbacks: low operating speed, large dimensions, and difficulty in changing spools. The installation and removal of spools require additional equipment, such as a crane or hoist, which increases setup time. The low rotation speed of the cages is due to the heavy weight of the spools, their distance from the cage's axis, and the presence of the backtwist device.

These machines are actively used in the process of main stranding (system 1+6+12+18), where backtwist is necessary to prevent deformation of the insulation. Without backtwist, the core is additionally twisted around its axis, which can cause wrinkles or indentations on the insulation layer, especially when working with large cross-section cores and thick insulation.

Rigid-frame stranding machines

Rigid-frame stranding machines are a simplified version of cage-type machines, without the backtwist device. This makes their design more compact and increases the twisting speed, as the spools with the workpieces are positioned closer to the rotation axis. These machines are designed for stranding compacted round and sector-shaped conductors of power cables, as well as for applying wire armor. The standard design includes a 6+12+18+24+30 system.

Each cage is equipped with a side automatic loading device, which significantly reduces the machine setup time compared to cage-type counterparts. Tension control is carried out either mechanically (with a brake band) or pneumatically.

Pneumatic braking: tension is regulated through the touch screen based on the wire diameter and the material of the workpiece. The air pressure is adjusted proportionally to the activation of the spool, maintaining constant tension.

The supply of compressed air can be carried out in two ways:

- through an expansion tank installed for each row of spools. Before starting the line, the operator fills the tank with air, and during operation, the pressure gradually decreases. Refilling the tank requires stopping the line.

- using a rotary pneumatic clutch, which transfers air from the main expansion tank to the rotating cage.

Rigid-frame machines can be equipped with a pre-twisting device combined with compacting rollers. Pre-twisting plays a key role in forming a round core, which is essential for producing high-quality cable and wire products.

Thus, cage-type machines are suitable for complex operations requiring backtwist, while rigid-frame machines are ideal for fast stranding of compacted conductors and cable armoring.

Compaction of the conductive core (CC)

Compaction of the conductive core (CC) is a technological process aimed at reducing the weight and dimensions of cable and wire products. It is performed during the stranding stage using compacting rollers or dies. The compaction unit is positioned after the calibrating die in the stranding line. The process takes place during stranding with the help of compacting rollers or dies, which are located in the compaction unit after the calibrating die. The sizes of the rollers and dies are selected based on the conductor material (copper, aluminum), its quality, and structural parameters such as the number of layers and the stranding pitch.

During the compaction process, the conductor passes through rollers or dies that shape its geometry and density. For round conductors, oval and round-profile rollers are used, while for sector-shaped conductors, rollers with the corresponding profile are applied. The rounding of the edges of sector-shaped conductors (radius ≥ 1 mm) reduces the concentration of the electric field, preventing insulation breakdowns.

The result of compaction is a reduction in the conductor diameter by 20–25%, an increase in the structural density, and improved operational characteristics of the cable. This makes the product more compact, lightweight, and resistant to stress, which is especially important for modern power systems. Thus, compaction of the conductive core (CC) is a key stage in production, ensuring high quality and reliability of cable and wire products.

Schematic representation of compacted conductive cores (CC):

Sectoral compacted conductive core (CC) for a 3-core cable, 120° angle

Segmental compacted conductive core (CC) for 2-core cables

Sectoral compacted conductive core (CC) for a 4-core cable, 94.5°-100° angle

Sectoral compacted neutral conductive core (CC) for a 4-core cable, 48°-60° angle

Compaction of round conductive core

A round non-compacted twisted conductive core (CC) typically passes through four pairs of rollers:

- the first vertical pair of oval-profile rollers

- he second horizontal pair of oval-profile rollers

-the third vertical and fourth horizontal pair of round-profile rollers.

To calculate the size of a sectoral conductive core, the following data is required:

Radius of the edge rounding of the sector r, thickness of the phase insulation, Δiz, cross-sectional area of the conductor, S, angle of the sector, β (for a segmental CC, the angle is 180°)